The document discusses cellular respiration. It begins by explaining that respiration uses energy stored in molecules like carbohydrates and fats to produce ATP through pathways like glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation in the mitochondria. It then discusses the specific multi-step pathways of aerobic cellular respiration, the role of coenzymes, and the structure and function of mitochondria. It concludes by describing anaerobic respiration pathways like lactic acid fermentation in humans and alcoholic fermentation in yeast that can function without oxygen.

1. Biology – Communication, Homeostasis and Energy
Module 4 – Respiration
1) WHY DO LIVING ORGANISMS NEED TO RESPIRE?
Respiration is the process whereby energy stored in complex organic molecules
(carbohydrates, fats and proteins) is used to make ATP.
Energy exists as potential energy and kinetic energy. Moving molecules have kinetic
energy that allows them to diffuse down concentration gradients. Chemical potential
energy is present in large organic molecules.
We need energy to allow all of our biological processes in our cells to take place.
These processes are known as metabolism collectively. Building large molecules
are described as anabolic processes and breaking them down are known as
catabolic processes.
Energy comes from sources such as sun light, and it cannot be created or destroyed,
just converted to a different form. Plants, and other organisms known as
photoautotrophs, can synthesis large biological molecules such as glucose. This is
an example of converting light energy from sunlight to chemical energy stored in the
large molecule. Respiration of these organic molecules releases this chemical
energy.
ATP is a phosphorylated nucleotide, which, when hydrolysed, releases 30.6kJ of
energy per mole. It is continually being hydrolysed and resynthesised (by respiration)
during cellular processes.
2) COENZYMES
The respiration of the main respiratory substrate, glucose, can be described in four
stages, all of which involve metabolic pathways involving enzymes and substrates:
Glycolysis – occurring in the cytoplasm, this involves the breakdown of a glucose
molecule to 2 pyruvate molecules. It occurs in both aerobic and anaerobic
respiration.
The Link Reaction – this happens in the matrix of mitochondria. Pyruvate is
dehydrogenated and decarboxylated in order to convert it to acetate.
Krebs Cycle – also takes place in the matrix of mitochondria. Acetate is also
decarboxylated and dehydrogenated.
Oxidative Phosphorylation – takes place on the inner membrane of the mitochondria,
and ADP is phosphorylated to ATP.
In all stages of respiration, apart from oxidative phosphorylation, hydrogen atoms are
removed from substrates during oxidation reactions. They are added to other

2. molecules in reduction reactions. Enzymes on their own are not very efficient at
carrying out these reactions, so require coenzymes to work effectively. Coenzymes
such as NAD accept the hydrogen atoms, and then carry them to the mitochondrial
matrix to be involved in the process of oxidative phosphorylation.
NAD – This coenzyme is required to carry hydrogen atoms with their electrons to the
cristae of the mitochondria where the NAD will be oxidised and its associated
hydrogen atoms will be lost.
Coenzyme A – This coenzyme carries ethanoate or acetate groups from the link
reaction the Krebs cycle.
3) GLYCOLYSIS
Glycolysis occurs in both pro- and eukaryotic cells. It occurs in the cytoplasm and
involves the breakdown of glucose into pyruvate through the process of many
enzyme-catalysed reactions of glucose and other intermediate substrates.
Glycolysis:
- One molecule of ATP is hydrolysed, and its inorganic phosphate group binds
to glucose at carbon 6, forming glucose 6-phosphate.
- This is then changed to fructose 6-phosphate.
- Another ATP molecule is hydrolysed, and this time the inorganic phosphate
group is bound to fructose 6-phosphate at carbon 1, forming fructose 1,6-
bisphosphate.
- The energy from the hydrolysis of this ATP molecule activates the hexose
sugar and keeps it within the cell.
- Hexose 1,6-bisphosphate is then split into two molecules of triose phosphate,
a 3-carbon compound.
- The oxidation of triose phosphate
reduces 2 NAD molecules and two
molecules of ATP are formed at this
stage by substrate level
phosphorylation.
- Four more reactions convert each TP
molecule to a molecule of pyruvate.
This process causes another two
molecules of ADP to become
phosphorylated to ATP by substrate
level phosphorylation.
Glycosis can be shown in simple terms by
this equation:
Glucose  2 x Pyruvate + 2NADH + 2ATP (net)

3. 4) STRUCTURE AND FUNCTION OF MITOCHONDRIA
Structure:
- All mitochondria have an inner and outer membrane, known as an envelope.
- The inner membrane is folded into cristae, which increases the surface area.
- The mitochondrial matrix is enclosed by the inner membrane. It’s semi-rigid,
gel-like and it consists of a mixture of proteins and lipids. It also contains
mitochondrial DNA, mitochondrial ribosomes and associated enzymes.
The matrix is where the link reaction and the Krebs cycle take place. It contains:
- Enzymes necessary for these reactions.
- Molecules of NAD
- Oxaloacetate – a 4-carbon compound that accepts acetate from the link
reaction
- Mitochondrial DNA, for synthesis of enzymes and other mitochondrial
proteins.
- Mitochondrial ribosomes, see above.
The inner membrane is impermeable to most small ions, including protons (hydrogen
ions). It is also folded into numerous cristae to give it a large surface area and has
embedded in it many electron carriers and ATP synthase enzymes.
The outer membrane is similar to other organelles’ membranes, in that they allow
molecules to pass over it by facilitated or passive diffusion, active transport, etc.
5) THE LINK REACTION AND KREBS CYCLE
Decarboxylation and dehydrogenation of pyruvate to acetate are enzyme-catalysed
reactions. Pyruvate dehydrogenase removes hydrogen atoms from pyruvate and
pyruvate decarboxylase removes a carboxyl group, which eventually becomes CO2,
from pyruvate. The coenzyme NAD accepts the hydrogen atoms and coenzyme A
accepts acetate, forming acetyl coenzyme A. CoA carries acetate to the Krebs cycle.
Summarising the link reaction:
2 x Pyruvate + 2NAD + 2CoA  2NADH + 2 x acetyl CoA + 2CO2

4. The Krebs cycle takes place in the mitochondrial matrix. It oxidises the acetyl group
of acetyl CoA to two molecules of CO2. It also produces one molecule of ATP by
substrate-level phosphorylation, and reduces 3 molecules of NAD with 1 molecule of
FAD:
- The acetate is offloaded from CoA and joins with a 4-carbon compound,
known as oxaloacetate, forming 6-carbon citrate.
- Citrate is decarboxylated and dehydrogenated to form a 5-carbon compound.
This process also reduces NAD.
- The 5-carbon compound is decarboxylated and dehydrogenated to form a 4-
carbon compound. This process also reduces NAD.
- The 4-carbon compound is changed into another 4-carbon compound. During
this, ADP is phosphorylated to form ATP.
- The second 4-carbon compound is changed into another 4-carbon compound.
A pair of hydrogen atoms are removed and accepted by FAD, which is
reduced.
- The third 4-carbon compound is further dehydrogenated and regenerates
oxaloacetate, reducing another molecule of NAD.
For each molecules of glucose, in the link reaction and Krebs cycle, the following are
produced:
- 8 NADH
- 2 FADH2
- 6 CO2
- 2 ATP

5. 6) OXIDATIVE PHOSPHORYLATION AND CHEMIOSMOSIS
The final stage of aerobic respiration involves electron carriers embedded in the
inner mitochondrial membranes. These membranes are folded into cristae, thus
increasing the surface area for electron carriers and ATP synthase enzymes. NADH
and FADH2 are reoxidised when they donate hydrogen atoms, split into protons and
electrons, to the electron carriers. The first electron carrier to accept electrons from
reduced NAD is a protein complex, complex I, called NADH – coenzyme Q
reductase. The protons are driven into the intermembrane space as electrons flow
along the electron transport chain, so a proton gradient builds up in the
intermembrane space.
These protons can only flow back into the matrix through an ATP synthase enzyme.
As the protons flow back through, the drive the rotation of part of the enzyme and
join ADP to inorganic phosphate to form ATP. The electrons are accepted by
oxygen, the final electron acceptor, which eventually forms water using hydrogen
ions.
Theoretically, 32 molecules of ATP should be yielded per molecule of glucose
respired, however, this is rarely achieved, as:
- Some protons leak across the mitochondrial membrane, reducing the
potential energy of the protons to make ATP.
- Some ATP produced is used to transport pyruvate into the mitochondrial
matrix.
- Some ATP is used for the shuttle to bring hydrogen from reduced NAD made
during glycolysis into the mitochondria.

6. 7) ANAEROBIC RESPIRATION IN MAMMALS AND YEAST
Since oxygen is the final electron acceptor in oxidative phosphorylation, in the
absence of oxygen, the electron transport chain cannot function so the Krebs cycle
and the link reaction also stop. The NADH from oxidation of glucose during
glycolysis must continually be reoxidised so that ATP can continue to be produced
through anaerobic respiration.
In humans, lactate fermentation occurs under anaerobic conditions. Essentially,
pyruvate and 2H from NADH react, to form lactate in an enzyme-catalysed reaction
(lactate dehydrogenase).
Pyruvate + NADH  Lactate + NAD+
The lactate diffuses out of the cell into the blood stream and is carried to the liver.
When more oxygen is available (made so by “oxygen debt”) the lactate can be
converted back to pyruvate where it can enter the link reaction and Krebs cycle. The
reduction in pH due to lactate inhibits enzyme activity so muscle fatigue occurs as a
result of this.
In organisms like fungi, such as yeast, alcoholic fermentation occurs. Under
anaerobic conditions, pyruvate is decarboxylated to ethanal. It is a reaction catalysed
by the enzyme pyruvate decarboxylase. Ethanal accepts hydrogen atoms from
reduced NAD which reduces it to ethanol.
Pyruvate + NADH  Ethanal +NADH + CO2  Ethanol + NAD+ + CO2
8) RESPIRITORY SUBSTRATES
A respiratory substrate is any organic substance that can be used for respiration. It
needs to provide hydrogen atoms for the reduction of NAD and FAD and for
subsequent use in oxidative phosphorylation.
Carbohydrates, i.e. sugars, such as fructose or galactose, can be converted to
glucose for respiration. Disaccharides can by hydrolysed to from glucose and other
monosaccharides that can then be converted to glucose. Polysaccharides (glycogen
in animals, starch in plants) can be hydrolysed to glucose for respiration. All hexoses
have the same formula (C6H12O6) and so release the same amount of energy per
mole, i.e. they release the same amount of hydrogen atoms.
- The theoretical maximum yield for glucose is 2870 kJ per mol.
- It takes 30.6 kJ to produce 1 mol ATP.
- So, theoretically, the respiration of 1 mol of glucose should produce nearly 90
mol ATP.
- The actual yield is around 30 mol ATP, as the remaining energy is released
as heat, helping to maintain a suitable temperature for enzyme activity.

7. Amino acids in excess from the digestion of dietary protein can be deaminated to
keto acids. The keto acids can be converted to glycogen or fat as an energy store.
Both of these energy stores can be broken down to glucose for respiration.
Amino acids can also be directly respired. Under conditions of starvation, fasting or
prolonged exercise, muscle protein is hydrolysed to amino acids. Different amino
acids can be converted to either pyruvate, acetyl CoA, or intermediates in the Krebs
cycle. Thus, amino acids can only be respired aerobically.
The number of hydrogen atoms per mole accepted by NAD and then used in
oxidative phosphorylation is slightly more than the number of hydrogen atoms per
mole of glucose, so proteins release slightly more energy than equivalent masses of
carbohydrate.
Lipids in the form of triacylglycerol (triglycerides) are an energy store.
Triacylglycerols are formed in a condensation reaction between glycerol and three
fatty acid chains.
When energy is required, the triglyceride can be hydrolysed back to glycerol and
three fatty acid chains in a reaction catalysed by the enzyme lipase. The glycerol can
be converted to glucose for respiration. The fatty acids can be oxidised to release
more energy in a process called beta oxidation.
A fatty acid must first be activated by converting it to fatty acyl CoA. This requires a
molecule of ATP and release of energy from hydrolysis of two bonds to form AMP, Pi
and Pi. Thus, the equivalent of two molecules of ATP has been used.
The fatty acyl CoA then goes through a series of reactions in the mitochondrial
matrix resulting in the release of 2 carbons as acetyl CoA. Each time a bond is
broken in the fatty acid chain to release acetyl CoA, a molecule of NADH is formed
and a molecule of FADH2 is formed.